Various types of recordable and/or erasable media have been used for many years for data storage purposes. Such media may include, for example, magnetic tapes or disks in systems having a variety of configurations.
Magneto-optical ("MO") systems exist for recording data on and retrieving data from a magnetic disk. The process of recording in a magneto-optical system typically involves use of a magnetic field to orient the polarity of a generalized area on the disk while a laser pulse heats a localized area, thereby fixing the polarity of the localized area. The localized area with fixed polarity is commonly called a pit or mark. Some encoding systems use the existence or absence of a pit or mark on the disk to define the recorded data as a "1" or "0", respectively.
When recording data, a binary input data sequence may be converted by digital modulation to a different binary sequence having more desirable properties. A modulator may, for example, convert m data bits to a code word with n modulation code bits (or "binits"). In most cases, there are more code bits than data bits--i.e., m&lt;n.
Most if not all disk drive systems use run-length-limited ("RLL") modulation codes, such as RLL 2/7/1/2 or RLL 1/7/2/3 codes. Another family of modulation codes are group-coded recording ("GCR") codes, such as GCR 0/11/8/9 or GCR 0/3/8/9 codes (sometimes abbreviated as GCR 8/9). The numbers appended to the names of particular codes typically refer to certain encoding constraints, such the relationship between bits and flux reversals, or the minimum and maximum number of contiguous binits possible without flux transitions. For example, a commonly used encoding system for pit-type recording is the RLL 2/7/1/2 code which constrains the recorded information to have a minimum of two and a maximum of seven zeroes between ones. In general, RLL recording provides a relatively high data-to-pit ratio but may not, however, in many circumstances allow for high data storage densities because amplitude and timing margins deteriorate very rapidly as frequency is increased.
A GCR 0/3/8/9 code, on the other hand, not only requires nine flux reversals for every eight data bits but further requires a minimum of no zeroes and a maximum of three zeroes between ones.
The density ratio of a given recording system is often expressed according to the equation (m/n).times.(d+1), where m and n have the definitions provided above, and d is defined as the minimum number of zeroes occurring between ones. Thus, the RLL 2/7/1/2 code has, according to the above equation, a density ratio of 1.5, while the GCR 0/3/8/9 code has a density ratio of 0.89.
For reading data in an MO system, a focused laser beam or other optical device is typically directed at the recording surface of a rotating optical disk such that the laser beam can selectively access one of a plurality of tracks on the recorded surface. The rotation of the laser beam reflected from the recorded surface may be detected by means of Kerr rotation. A change in Kerr rotation of a first type, for example, represents a first binary value. A change in Kerr rotation of a second type represents a second binary value. An output signal is generated from the first and second binary values occurring at specified clock intervals.
Although there has been a continual demand for disk systems capable of storing increasingly higher data densities, the ability to achieve high data storage densities has met with several limitations. As a general matter, the reasonable upper limit for data density is determined in part by reliability requirements, the optical wavelength of the laser diode, the quality of the optical module, hardware cost, and operating speed. Maximum data densities are also affected by the ability to reject various forms of noise, interference, and distortion. For example, the denser that data is packed, the more intersymbol interference will prevent accurate recovery of data. Moreover, because the technology for many intermediate and high performance optical disk drives has been limited by downward compatibility constraints to older models, signal processing techniques have not advanced as rapidly as they might have otherwise.
When attempting to recover stored data, existing read channels of magneto-optical and other types of disk drives commonly suffer from a number of problems due to the unintended buildup of DC components in the read signal. One cause of DC buildup results from the recording of asymmetric data patterns over a number of bytes or data segments. A symmetric data pattern may be considered as one having an average DC component of zero over a region of interest. Because sequences of recorded bits may be essentially random in many modulation codes, however, localized regions of recorded data having particular patterns of 1's and 0's will produce an unsymmetric read signal having unwanted DC components. Because the data patterns vary over time, the level of DC buildup will also vary, causing wander of the DC baseline, reduction of threshold detection margins, and greater susceptibility to noise and other interference.
Undesired DC buildup is also caused by variance in pit size due to thermal effects on the writing laser or the storage medium. As the writing laser heats up, for example, the spot size may increase leading to wider pits or marks. When the recorded pits are read, variations in pit size will cause an unsymmetric input signal having DC components. Variation in pit size not only causes undesired DC buildup but also causes the relative locations of the data to appear shifted in time, reducing the timing margin and leading to possible reading errors.
Various attempts have been made to overcome the described problems. For example, various tape drive systems commonly use a DC-free code such as a 0/3/8/10 code, otherwise referred to simply as an 8/10 code. Because an 8/10 code requires 10 stored bits to yield 8 data bits, however, it is only 80% efficient which is a drawback when attempting to record high data densities.
Another method for handling DC buildup involves the use of double differentiation. This method typically involves detection of the peaks of a first derivative of the input signal by detecting zero-crossings of the second derivative of the input signal. Thus, the DC components are effectively filtered out. One drawback of this method is that differentiation or double differentiation can cause undesirable noise effects. A second drawback is that the method may decrease the timing margin to unacceptably low levels (e.g., by as much as 50 percent).
In another method for addressing DC buildup, the data to be stored is randomized prior to recording such that none of the data patterns repeat over a data sector. However, this method may not be acceptable by ISO standards and may lack downward compatibility with previous disk drive systems. As a further drawback to this method, de-randomizing the data may be complex.
Yet another method for controlling DC buildup involves the use of so-called resync bytes between data segments. This method generally involves the examination and manipulation of data before it is recorded in order to minimize DC buildup upon readback. Before recording, two consecutive data segments are examined to determine if the patterns of 1's and 0's are such as to cause positive DC, negative DC, or no DC components when read back. If, for example, two consecutive data segments have the same DC polarity, one of the data segments is inverted prior to being recorded on the medium. In order to stay within the constraints of the particular encoding system, however, a resync byte between the segments may need to be written so that the pattern of contiguous bits and flux reversals is proper. A drawback of such a method is that it will not necessarily reduce all DC buildup, and time constants must be determined such that the predictable DC buildup will not affect performance. Further, the method requires additional overhead including the examination of data segments to determine their relative polarity.
It would therefore be advantageous to have a method and device for reading stored data from a medium without suffering the undesirable effects of DC buildup, without creating unacceptable levels of noise or significantly reducing timing margins, without the requirement of large amounts of overhead or de-randomizing algorithms, while providing high data storage efficiency.